1. Field of the Invention
The present invention relates, in general, to a method for constructing a charge storage electrode of a semiconductor memory device in which a drain electrode and a source electrode are connected with a bit line and the charge storage electrode, respectively and, more particularly, to a method for constructing a charge storage electrode of a semiconductor memory device, capable of minimizing the cell area and maximizing the capacity of the semiconductor memory device.
2. Description of the Prior Art
In general, as the area of a unit cell is reduced according to the high integration of the semiconductor memory device, it is required that a charge storage electrode connected with a source electrode secure sufficient capacity of the semiconductor memory device, in order for the memory device to have sufficient sensitivity to the data and that a bit line be connected with the transistor.
In constructing a contact for a charge storage electrode that is connected with the source electrode and insulated from a gate electrode, the contact for the charge storage electrode has to be spaced from the gate electrode by a predetermined distance. That is, in order to construct the charge storage electrode, a contact mask for the charge storage electrode is spaced apart from the gate electrode in consideration of the misalignment tolerance and critical dimension variation that may be generated during mask work, the thickness of insulating film that formed between the charge storage electrode and the gate electrode.
The prior art relating to the construction of charge storage electrode having a dual structure will be described next with reference to FIGS. 1A to 2C wherein reference numeral 1 designates a semiconductor substrate while reference numerals 2, 3, 4, 5, 5', 6, 7, 8, 9 and 10 designate a device separation insulating film, a gate oxide film, a gate electrode, a source electrode, a drain electrode, an interlayer insulating film, a first conductive material for charge storage electrode, a sacrificial film, a second conductive material for charge storage electrode and a spacer insulating film, respectively, For convenience, the description is continued to only the step in that the charge storage electrode is formed.
Referring initially to FIG. 1A, there is, in part, illustrated one conventional construction method of charge storage electrode having a dual structure. As illustrated in this figure, one conventional charge storage electrode is constructed by firstly sectioning a semiconductor substrate 1 into an active region and a device separation region by means of a device separation insulating film 2. Then, a gate electrode 4 is formed on the active region and the device separation insulating film 2, respectively, followed by the formation of source/drain electrodes 5, 5' in the semi conductor substrate. Thereafter, over the resulting structure, there is entirely formed an interlayer insulating film 6, which is subsequently coated with a first conductive material for charge storage electrode 7 covered with a sacrificial film 8.
FIG. 1B illustrates the formation of a second conductive material for charge storage electrode 9. For this, using a contact mask for charge storage electrode (not shown), an etch process is applied to the sacrificial film 8, the first conductive material for charge storage electrode 7 and the interlayer insulating film 6 to form a contact hole on a predetermined portion of the source electrode 5. Thereafter, the second conductive material for charge storage electrode 9 is entirely deposited over the resulting structure.
Referring now to FIG. 1C, there is illustrated the one conventional method for constructing a charge storage electrode in a semiconductor memory device, continued from FIG. 1B. The second conductive material for charge storage electrode 9 is subjected to the treatment of etch to remove a predetermined portion thereof by use of another contact mask for charge storage electrode (not shown). The resulting second conductive material for charge storage electrode 9 subsequently serves as an etching barrier which is used to etch both the sacrificial film 8 and the first conductive material for charge storage electrode 7, in order to form a charge storage electrode 20 having dual structure, connected with the source electrode 5.
The above-mentioned conventional method can provide a charge storage electrode of dual structure that allows the capacity of the semiconductor memory device to increase. In constructing the charge storage electrode by use of the conventional method, however, it is restrained to diminish the contact for charge storage electrode into below some size due to the photo developing technique being used. Also, since the charge storage electrode according to the one conventional method has to be spaced out from the lower gate electrodes 4 by a predetermined distance, the area of the charge storage electrode contact can not be reduced. Accordingly, there are many problems in fabricating a highly integrated semiconductor memory device with the one conventional method.
Turning now to FIGS. 2A through 2C, there is illustrated another conventional construction process for forming a charge storage electrode a semiconductor memory device. This conventional charge storage electrode structure adopts a spacer insulating film at the side wall of the charge storage electrode contact, in order to reduce the area of the charge storage electrode contact. However, the first conductive material for the charge storage electrode is disconnected from the second conductive material due to the spacer insulating film, so that the charge storage electrode is not formed into a dual structure.
In detail, a semiconductor substrate 1 is sectioned into an active region and a device separation region by means of a device separation insulating film 2, as shown in FIG. 2A. Thereafter, a gate electrode 4 is formed on the active region and the device separation region, respectively and source/drain electrodes 5, 5' are formed in the active region. Over the resulting structure, there is entirely formed an interlayer insulating film 6 in a predetermined thickness, which is then coated with a first conductive material for charge storage electrode 7 covered with a sacrificial film 8. Using a contact mask for charge storage electrode (not shown), an etch process is applied to the sacrificial film 8, the first conductive material for charge storage electrode 7 and the interlayer insulating film 6, in due order, in order to form a contact hole for charge storage electrode on the source electrode 5. Thereafter, a spacer film 10 is formed at the side wall of the contact hole for the charge storage electrode 7.
At this time, as the sacrificial film 8, there is employed those that show large values of etching selection ratio in relation to the spacer insulating film 10 formed at the side wall of the contact hole of charge storage electrode. For example, while the spacer insulating film 10 is normally an undoped silicate glass (hereinafter "USG"), the sacrificial film 8 is either a phospho-silicate glass (hereinafter "PSG") film or a boro-phospho-silicate glass (hereinafter "BPSG") film. The etching selection ratio of USG film to PSG (or BPSG) film is at least 10:1 in a solution containing HF and H.sub.2 O or in a solution containing NH.sub.4 OH, H.sub.2 O.sub.2 and H.sub.2 O.
The spacer insulating film 10 serves not only to reduce the size of the contact for charge storage electrode, but also to make the contact for charge storage electrode be spaced apart from the gate electrodes by a predetermined distance, attributing to diminishing the area of the cell.
As shown in FIG. 2B, a second conductive material for charge storage electrode 9 is formed over the sacrificial film 8 and in the contact hole, so that it is connected with the source electrode 5. In the meanwhile, the second conductive material for charge storage electrode 9 is insulated from the first conductive material for charge storage electrode 7 by the spacer insulating film 10 formed at the side wall of the contact hole.
Finally, as shown in FIG. 2C, the second conductive material for charge storage electrode 9 is subjected to the treatment of etch to remove a predetermined portion thereof by use of another contact mask for charge storage electrode (not shown). The resulting second conductive material for charge storage electrode 9 subsequently serves as an etching barrier which is used for etching both the sacrificial film 8 and the first conductive material for charge storage electrode 7, in order to form a charge storage electrode 20.
As briefly mentioned above, another conventional method for the construction of charge storage electrode is characterized by forming a spacer insulating film at the side wall of the contact for the charge storage electrode, in order to reduce the area of the cell, thereby performing high integration of the semiconductor memory device. According to the conventional method, there is provided a charge storage electrode diminished in the area of contact for the charge storage electrode, thus attributing to reducing the area of the cell. In this conventional method, however, the first conductive material for the charge storage electrode is disconnected from the second conductive material for charge storage electrode by the spacer insulating film, causing the charge storage electrode not to have a dual structure. Therefore, it is difficult to sufficiently secure the capacity of the semiconductor memory device employing the charge storage electrode constructed by the conventional method.